The likely source of the respiratory disease Sars is the horseshoe bat, a new study suggests.

Researchers found a virus closely related to the Sars coronavirus in bats from three regions of China.

Writing in the journal Science, they say the virus may have needed to infect another animal such as the civet before it could transmit to humans.

They suggest that live horseshoe bats are kept out of markets until the transmission path is fully understood.

The Sars (Severe Acute Respiratory Syndrome) outbreak in 2002/3 caused about 770 deaths, and economic damage estimated in billions of dollars. Centred on east Asia with origins in southern China, fatalities occurred as far afield as Canada.

The virus we found is 92% similar to the human Sars virus

Zhengli Shi, Chinese Academy of Sciences

Schools and businesses closed, international trade and travel were restricted; and for a time, until basic public health measures curtailed the outbreak, it seemed as though the next major global disease of humanity had emerged.

But emerged from where? In May 2003, the suggestion was that the virus responsible had entered the human population from civets, animals eaten in wildlife restaurants and butchered in live animal markets in southern China.

The World Health Organization (WHO) endorsed this link early in 2004, an announcement which led authorities in China to embark on a culling programme which saw an estimated 10,000 civets killed, as well as other animals suspected of harbouring Sars, such as badgers and raccoons.

Immunity clue

But for some time, the prevailing theory among scientists has been that civets were not the original source, or reservoir, of infection.

One clue is that they appear to have little immunity, and become seriously ill; whereas species which harbour pathogens for a long period of history usually adapt to them.

The bats are on sale in Chinese markets

So where did the Sars virus, labelled Sars-CoV, come from?

One theory named birds; but earlier this month, researchers at Hong Kong University found cause to suspect bats. In a Hong Kong bat species they found a virus closely related to that found in Sars patients.

Now an international collaboration between scientists in China, Australia and the US has gone further, and identified a Sars-like virus in three species of bats from mainland China.

"The virus we found is 92% similar to the human Sars virus," said Zhengli Shi, from the Institute of Zoology at the Chinese Academy of Sciences, Beijing.

"Why it is there in these bats, why it can infect just these species, we are not sure - it is a story we want to discuss," she told the BBC News website.

All three species of bat in which Dr Shi's group found the Sars-like coronavirus, dubbed SL-CoV, are horseshoe bats of the genus Rhinolophus, as is the species identified in the Hong Kong study.

Civets still implicated?

Genomic analysis suggests that the bat coronaviruses found by this group and by the Hong Kong team are very alike, and that both are closely related to the human and civet forms. The major differences lie in genes which relate to the binding of virus particle and host cell.

"This virus, we are sure, cannot infect humans," said Zhengli Shi.

One of the big questions is, then, how the virus jumped from bats to humans - and whether in the body of an intermediary, such as the civet, it can adapt in such a way that it can then infect a human.

Civets: The "amplifier host"?

"At the moment we don't know," said Peter Daszak, director of the Consortium for Conservation Medicine in New York, US, who was also involved in the study.

"But we can make a comparison with other viruses - for example, we don't know what the original host is for Ebola, but it appears to get into chimpanzees first, and then into humans.

"Nipah virus, which emerged in Malaysia in 1998 and 99, we believe has fruit bats as the reservoir, but it had to go into pigs before it could infect humans."

So civets could be an "amplifier host" for Sars. If they are, one suggestion, according to Peter Daszak, is to keep them away from horseshoe bats.

"In the east Asian region, we need to face up to high-risk behaviours," he said, "and in this situation, bringing these species into live markets, butchering and eating them and using them in medicines, is a high-risk behaviour."

Solving the jigsaw

WHO spokesperson Dick Thompson told the BBC News website: "We see this as another piece of the Sars jigsaw.

"There's an unfinished agenda for Sars, and clearly we need to understand the disease ecology better."

The Chinese team plans to examine the possible transmission path of the virus more closely.

"We will change some amino-acid sequences in the virus we have identified," said Zhengli Shi, "and see if can infect humans."

Confirming horseshoe bats as the source of Sars would carry implications for future public health research and policy.

"These bats have a wide distribution in Europe and Asia," said Peter Daszak, "and what we don't know, and need to know urgently, is the distribution of the Sars-like virus in these bats.

"On a wider scale, we need surveillance of wildlife to look for possible new diseases, and to identify changes in the environment, human behaviour and demography which drive the emergence of these diseases; because almost every new disease which has emerged recently has been driven by changes in land use.

"The last thing we should do is to take it out on the bats, because the evidence suggests that they have carried this coronavirus for thousands, perhaps millions, of years; only recently has it emerged in a big way, and it was human behaviours that made the difference."